In situ microscopy (ISM) involves the use of a direct-light microscope with a measuring chamber, integrated in a 25-mm stainless
steel tube, as well as two CCD-cameras and two frame-grabbers. The data obtained are processed by an automatic image analysis
system. The microscopic examination of the liquid is performed in the measuring chamber, which is situated near the front
end of the sensor head. ISM mainly allows for on-line microscopic observation of microorganisms during fermentations in bioreactors
(40). Applications include in-line monitoring of Hansenula anomala culture in a batch fermentation process; real-time monitoring
of hybridoma cells concentration in a stirred bioreactor; measurement of the level of colonization of fibroblasts (murine
cell line NIH-3T3) on microcarrier surfaces during cultivation; detection of cell viability without conventional staining
techniques; and on-line monitoring of enzyme carriers to determine their mechanical stability in the bioreactor (41-45).
Microwave resonance technology (MRT) is a noninvasive and nondestructive method of evaluating the moisture content of analytes
using a microstrip waveguide with a resonance in the 300 MHz to 3 GHz range. It has been used for continuous moisture measurement
in pharmaceutical granules and for determining moisture, temperature, and density of the granules to improve process understanding
in fluid bed granulation (46).
Terahertz technology has a frequency gap from 100 GHz to 30 THz in the electromagnetic spectrum, which is in between microwave
and infrared and has found a variety of applications in biology, medical science, quality control of food, pharmaceutical,
and agriculture products, as well as global environment monitoring. Terahertz time domain spectroscopy (THz-TDS) also has
been applied to many materials, including biomolecules, medicines, cancer tissue, DNA, proteins, and bacteria (47). It enables
3D imaging of structures and materials, and the measurement of the unique spectral fingerprints of chemical and physical forms,
which is especially relevant in 3D imaging of tablets (48) and estimation of concentrations of the various excipients (49).
Photoacoustic spectroscopy (PAS) is based on the absorption of electromagnetic radiation by the molecules in the sample. The
nonirradiative collisions of molecules in sample lead to warming of the sample matrix, which gives a pressure fluctuation
because of thermal expansion that can be detected in the form of acoustic waves (50). These waves propagate through the volume
of the gas to the detector (microphone, piezoelectric transducers, or optical method) where a signal is produced. This detector
or microphone signal, when plotted as a function of wavelength, will give a spectrum proportional to the absorption spectrum
of the sample (51). Applications include on-line and nondestructive monitoring of growth, thickness, and detachment of biofilms
in waste water treatment plants and unraveling the influence of various process parameters (such as pH, flow conditions) on
the structure and stability of a biofilm (52).